Lyme disease (LD) is a tick-borne, multi-system, infectious disorder caused by the extracellular spirochetal bacterium Borrelia burgdorferi (Bb). Since Bb lacks orthologs of known exotoxins or the specialized secretory machinery required for the delivery of noxious molecules into host cells, it is widely believed that the distinctive inflammatory clinical signs and symptoms associated with the disease result from the host's innate and co- evolving adaptive immune responses to the bacterium. Monocytes and macrophages are considered to be critical cellular elements of the innate immune response to the spirochete. For more than a decade, Bb- mediated cell activation was thought to occur chiefly as a result of the interactions of the spirochete's abundant outer membrane-associated lipoproteins with CD14 and Toll-like receptors (TLR) 1/2 on the surface of these cells. More recently, we have provided extensive evidence that phagocytosis of live spirochetes, by either human monocytes or murine macrophages, generates a more intense and far broader inflammatory response than can be attributed to lipoprotein-mediated, cell surface TLR1/2 activation. Phagocytosis of intact Bb not only enhanced TLR2-mediated, NF-B-dependent responses, it also induced TLR2-independent responses, most notably transcription of interferon- (IFN-) and type I interferon-stimulated genes (ISGs). Recent evidence from the investigator's laboratory suggests that the TLR2-independent responses are MyD88-dependent and occur via TLR8. Although intracellular bacterial pathogens have a well recognized ability to induce type I IFNs, they usually have to escape the phagosomal vacuole and engage cytosolic receptors in order to do so. Bb, on the other hand, has the ability to induce these same cytokines despite its inability to escape certain destruction within the phagosome;we have designated this distinctive and under-studied cell activation process "phagosomal signaling". Based upon our combined experimental observations we have formulated the following central hypothesis: "Phagocytosis of Borrelia burgdorferi and degradation of Lyme disease spirochetes within phagosomes induces diverse pro- and anti-inflammatory signaling cascades in monocytes/macrophages through a cooperative interaction between TLR1/2 and TLR8". The experimental objectives proposed herein are in line with our long term goals of elucidating the mechanisms by which Bb triggers the inflammatory processes that cause tissue damage and/or that promote bacterial clearance during the course of LD. To accomplish our goals and address our central hypothesis, we have formulated the following Specific aims. In Aim 1 we will examine the mechanisms by which phagocytosis of Borrelia burgdorferi elicits TLR1/2 and TLR8 dependent signals in human monocytes and/or macrophages. In Aim 2 we will determine if components of internalized, degraded Borrelia burgdorferi leak out of phagocytic vacuoles to engage cytosolic PRRs. In Aim 3: To characterize MyD88 dependent and independent responses of monocytes and macrophages to Borrelia burgdorferi both in vitro and in vivo. PUBLIC HEALTH RELEVANCE: Lyme disease (LD) is a tick-borne infectious disorder caused by the spirochetal bacteria Borrelia burgdorferi (Bb), which has continued to increase in endemic areas and has spread geographically, paralleling the distribution of its primary vector, Ixodes ricinus complex, and the explosive growth in the white-tailed deer population. To study the inflammatory responses to the LD spirochete, the investigator will use a powerful stimulation model that more completely characterizes how the bacterium activates human immune cells, and will also take advantage of the large repertoire of available knockout mice to perform the experiments proposed herein. This combined translational-experimental approach differs extensively from existing conventional models that rely exclusively on the mouse system to understand the disease, and instead utilizes the mouse model to illuminate mechanistic aspects of the immune response to the bacterium, which can then be re-examined and compared by using human cells. The experimental objectives proposed herein are in line with our long term goals of elucidating the mechanisms by which Bb triggers the inflammatory processes that cause tissue damage and/or that ultimately eliminate the invading spirochete during the course of LD.